Among the different techniques for the preparation of metal deposits, electrodeposition (also known as electroplating) is particularly attractive because of its relatively inexpensive instrumentation, low temperature operation, and simplicity. A further advantage of the technique is the relatively straightforward control of the thickness and composition of the depositing layers through electrical quantities such as current passed and potential applied.
Electroplating has been employed in small scale as well as industrial processes. For example, electroplating of precious metals to improve the appearance of an article or to create special effects is well known. Electroplating is also employed to improve the corrosion resistance of corrosive substances by depositing thin surface films of corrosion resistant metals such as zinc, tin, chromium, nickel and others. Wear resistant and friction modifying coatings of nickel, chromium, titanium and other metals and their alloys are used to improve the wear resistance of bearing surfaces. Electroplating is also employed in the electronics industry to improve or modify the electrical properties of substrates such as contacts, printed circuits, electrical conductors, and other electrical items in which specific surface or surface-to-substrate conductive properties are desired. Distinct metals are often electroplated onto metal surfaces to improve soldering characteristics or to facilitate subsequent coating by painting or application of other adhering films such as plastics, adhesives, rubber, or other materials.
Although the electrodeposition of a single material has been extensively studied, the deposition of two or more metals by electrochemical methods is difficult because the conditions favorable for the deposition of one metal may differ substantially with those necessary for the deposition of the other. Factors including widely differing reduction potentials, internal redox reactions that can alter the oxidation states of the materials in solution, and species that are or become insoluble during the deposition can disrupt the process. Moreover, the nature of the electrodeposit itself is determined by many factors including the electrolyte composition, pH, temperature and agitation, the potential applied between the electrodes, and the current density. These issues are more prevalent as the complexity of the electrodeposit (and hence the number of species in solution) increases.
Complex electodeposited materials are desired in areas such as catalysis where the composition of the electrodeposit is critical to its catalytic activity. The discovery of new catalytic materials depends largely on the ability to synthesize and analyze new compounds. Given approximately 100 elements in the periodic table that can be used to make such catalysts, and the fact that ternary, quaternary and greater compositions are desired, an incredibly large number of possible compositions is generated. Taking each of the previously noted electrochemical issues into account for every possible electrodeposit and designing a synthetic strategy that can effectively cover phase space using traditional synthetic methodologies is a time consuming and laborious practice. As such, there exists a need in the art for a more efficient, economical, and systematic approach for the synthesis of novel materials and for the screening of such materials for useful properties. See, for example, copending U.S. patent application 08/327,513 filed Oct. 18, 1994, now U.S. Pat. No. 5,985,356, entitled "The Combinatorial Synthesis of Novel Materials" (published as WO 96/11878).
As an example of the utility of exploring phase space for more effective catalysts, one can consider the effect of changing the composition of the anode in a direct methanol fuel cell. A tremendous amount of research in this area has concentrated on exploring the activity of surface modified binary, and to a much lesser extent ternary, alloys of platinum in an attempt to both increase the efficiency of and reduce the amount of precious metals in the anode part of the fuel cell. Although electrodeposition was explored as a route to the synthesis of anode materials (e.g., F. Richarz et al. Surface Science, 1995, 335, 361), only a few compositions were actually prepared, and these compositions were made using traditional single point electrodeposition techniques. Such an approach becomes very inefficient when exploring and optimizing new multi-component systems. Recently, Mallouk, et al. reported work on combinatorial electrochemistry as a route to fuel cell anode materials (Science, 1998, 280, 1735), but this technique did not employ electrodeposition techniques.
The present invention provides a method to use electrodeposition to synthesize and evaluate large numbers of distinct materials in relatively short periods of time, significantly reducing the time consuming and laborious processes normally associated with a novel materials discovery program.